1. Technical Field
The invention is related to radio frequency (R.F.) inductively or capacitively coupled plasma reactors used in processing semiconductor wafers, and in particular to improvements therein for increasing the plasma ion density uniformity across the wafer surface.
2. Background Art
Inductively coupled plasma reactors are currently used to perform various processes on semiconductor wafers including metal etching, dielectric etching and chemical vapor deposition, as some examples. In an etch process, one advantage of an inductively coupled plasma is that a high density plasma is provided to permit a large etch rate independently of wafer bias, thereby permitting more control of the wafer bias to reduce device damage. For this purpose, the source power applied to the antenna and the bias power applied to the wafer pedestal are controlled separately. Separating control of the bias power and source power facilitates independent control of ion density and ion energy, in accordance with well-known techniques. To produce an inductively coupled plasma, the antenna is a coil inductor adjacent the chamber, the coil inductor being connected to the RF source power supply. The coil inductor provides the RF power which ignites and sustains the plasma. The geometry of the coil inductor can in large part determine spatial distribution of the plasma ion density within the reactor chamber.
One problem with such a plasma reactor is that the spatial distribution of the plasma ion density across the wafer surface is often non-uniform. This is a problem in a metal etch process, for example, because the etch rate and accumulation of electric charge on the device is affected by plasma ion density. Specifically, non-uniform plasma ion density distribution tends to render the etch rate non-uniform across the wafer and leads to device damage from charge build-up. As a result, the etch process is difficult to control, over-etching devices on some portions of the wafer and under-etching devices on other portions of the wafer, leading to reduced production yield.
One of the causes of non-uniform plasma ion distribution is the coil geometry and location. Another cause is the shape of the plasma itself, which is largely determined by the shape of the reactor chamber, particularly the reactor chamber ceiling.
Generally, the coil inductor of an inductively coupled plasma reactor is wrapped around the reactor chamber, although it does not necessarily conform to the shape of the reactor chamber walls. Necessarily, different regions of the wafer surface are displaced from the nearest coil windings by different distances and therefore experience different plasma ion densities.
Depending upon the shape of the reactor chamber ceiling, more plasma volume is located over the wafer center and less over the wafer edges, particularly in the case of a conical or hemispherical ceiling, for example. Accordingly, there tends to be inherent spatial non-uniformities in the ion flux density.
A different approach is disclosed in U.S. Pat. No. 4,948,458 to James Ogle in which a plasma reactor has a flat ceiling and a flat coil antenna overlying the ceiling. However, this approach has generally been found to provide no improvement in plasma ion density uniformity and moreover suffers from relatively large capacitive coupling in the plasma, hindering control of the plasma ion energy. A modification of that approach is disclosed in U.S. Pat. No. 5,368,710 to Chen et al., in which an attempt is made to adjust the plasma characteristics such as density by increasing the thickness of the dielectric chamber cover toward the center of the chamber. U.S. Pat. No. 5,346,578 discloses a reactor having an arcuate ceiling. However, such techniques are generally limited in their best applications to a relatively narrow window of process recipes.
Thus, there is a need for a plasma reactor which permits versatile optimization of plasma characteristics to optimize uniformity of plasma ion density distribution over a large process window.